Conceptos de Potencia Motor

8/13/2019 Conceptos de Potencia Motor

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Engine Power Concepts

PRODUCT INFORMATION

Stroke

Stroke

C/R


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CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Engine Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Engine Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

Brake Mean Effective Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9

Power Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-13

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

P R O D U C T I N F O R M A T I O N E N G I N E P O W E R C O N C E P T S


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Selling engines requires knowledge of their

capabilities and components. Potential

customers will have questions regarding

applications and engine design features. While

some of the terms used to describe various

engine parts or functions can sound complex,

they refer to processes or relationships which

are actually easy to understand most relate to

size or power. These general terms apply to all

piston engines Cat engines as well as the

engine in your car.


4

ENGINE POWER CONCEPTS


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Bore refers to the inside diameter of the

cylinders in an engine. The piston is slightly

smaller than the bore measurement because it

slides in the cylinder.

Stroke is the distance the piston travels in the

cylinder. The length of the stroke is determined

by the crankshaft radius also known as crank

throw (the distance from the centerline of main

bearing journal to centerline of connecting rod

bearing journal). This movement is controlled

by the shape of the crankshaft.

The connecting rod connects the crankshaft to

the piston. As the crankshaft rotates through

180 degrees, the connecting rod and the piston

move from the extreme bottom position (BC) to

the extreme top position (TC). The stroke then is

two times the crankshaft crank radius (C/R). The

crank radius is also the lever arm on which the

force from the piston acts to produce torque.

Displacement, or swept volume, per cylinder is

the volume of air a piston displaces as it moves

through one stroke. These terms are used

interchangeably. Both mean bore area times

stroke.

Bore Area = (3.14 x bore squared)/4

Displacement per Cylinder = Bore Area x Stroke

Engine Displacement = Displacement per

Cylinder x No. of Cylinders

If the bore diameter and stroke are in inches,

the displacement will be in cubic inches. If the

bore diameter and stroke are in centimeters,displacement is in cubic centimeters. 100 cubic

centimeters is one liter.

Compression Ratio is the ratio of volume in the

cylinder with the piston all the way down vs.

all the way up. If the minimum volume in the

cylinder with the piston at TC is one cubic inch

and the maximum volume with the piston at BC

is 10 cubic inches, the compression ratio is 10:1.

Automotive gasoline engines have

compression ratios between 7:1 and 12:1.

Diesel engines have compression ratios

between 13:1 and 24:1. Generally, larger

diesel engines have the lower compression

ratios.


ENGINE SIZING

displacementor

swept

volume

Stroke

Stroke

C/R

C/R


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Engine speed (the number of revolutions made

by the crankshaft in one minute) is measured in

rpm (revolutions per minute).

Torque is the twist on a shaft resulting from a

force applied perpendicular at a lever arm. Its

units are force (pounds) times distance from the

center of the rotating shaft (feet). Thus, 100

pounds applied at a lever arm of 2 feet results in

200 lb-ft torque. Equal torques can be produced

by a large force applied at a short lever arm or

a small force applied at a long lever arm. The

torque from one-pound force applied at a

10-foot lever is the same as from a 10-pound

force applied at a one-foot lever, etc.

In an engine, pressure is applied to the top of the

piston from expansion of an ignited air and fuelmixture. This pressure results in a force from the

piston applied at the crank radius through the

connecting rod. The resulting torque causes the

crankshaft to rotate.

By definition, work is force applied for a

distance, or in the case of a rotational situation,

work is torque applied through an angle. Power

is work performed per unit of time.

An engine producing 1000 lb-ft torque at 2000

rpm, through transmission gearing can produce2000 lb-ft torque at 1000 rpm assuming no

efficiency losses through the transmission, for

example. An increase in torque is achieved at the

expense of speed. The power in both cases is the

same. To increase engine power we strive to

increase torque (lb-ft) or speed (rpm) or both.

2ft

2lbsT

orque

4 lb-ft


6

ENGINE POWER

rpm Torque Lever arm

Torque = 10 lb-ft

10 ft

1 ft

1 lb

10 lbs


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The most common unit of engine power in the

U.S. is horsepower (hp). Originally, this unit

was derived by what an average horse could do.

Rigged up with a pulley system, an average

horse could lift 33,000 pounds one foot off the

ground in one minute.

Because power takes into account engine torqueoutput as well as engine speed, it is a convenient

unit used to compare engine size.

Though horsepower is an accepted unit to rate

engines, each application must be considered

individually. The engine ratings can be:

Power that can be produced continuously

Power that can be produced for a given

time period, (generally one hour) followed

by an equal time period at a lower rating Power that the engine can deliver for very

short times, such as five minutes

Variables influencing power rating are:

Temperature of the air

Temperature of the fuel

Barometric pressure

Humidity

Heat content of the fuel

The total horsepower actually developed on the

pistons is called indicated horsepower. It is

greater than the power measured at the engine

flywheel by the horsepower required toovercome frictional losses in the bearings, piston

rings, etc. as well as operating satellite systems

such as fuel, oil, and water pumps. The

difference between indicated horsepower and

flywheel horsepower is called friction

horsepower.

The friction horsepower of an engine can be

determined in the laboratory by motoring the

engine with an electric motor. In this test the

engines fuel rack is at shut-off. The electric

power required to motor the engine at anygiven speed is the engine friction horsepower

at that speed.


ENGINE POWER


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Indicated horsepower less frictional

horsepower equals brake horsepower.

BMEP is a value referring to the constant

pressure which would have to exist in a cylinder

during its power stroke to produce the same

horsepower at the flywheel, as actually exists.

Pressure within the cylinder varies considerably.

A rough indication of that pressure is shown

above. You see that the pressure acting on the

piston varies considerably during the power

stroke. The mean or average pressure whichwould produce the same brake horsepower is

the BMEP.

Another way of viewing BMEP is that it

measures how effectively an engine uses its

piston displacement to produce torque.

The higher the BMEP, the greater the

torque per unit of displacement.

BMEP can only be compared between

4-cycle engine to 4-cycle engine and 2-cycle

engine to 2-cycle engine.

Over the years, BMEP has become known

as a measure of engine life, however, it is

NOT.

BMEP gives a fair indication of mechanical

stresses within the engine, but in no way is

indicative of thermal loads.

Example: One engine operating at the same

speed (1800 rpm), but with varying turbocharger

boost and amount of aftercooling.

Column 1 shows a naturally aspirated engine

producing 100 hp at a BMEP of 84

Column 2 light turbocharging greatly

increases the air inlet temperature, but raises

horsepower to 134 and BMEP to 119; fuel

consumption has decreased 6% to .402 lbs./bhp hr.,

but internal pressures have increased 44%;


8

BRAKE MEAN EFFECTIVE PRESSURE

compression

pressu

re

power exhaust intake

+

o

compression

pressure

power exhaust intake

+

o

BMEP

Example Engine (1800 rpm)


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because air inlet temperature has increased 171,

thermal loading has increased 19%.

Column 3 an engine with the same degree of

turbocharging as in column 2, but with moderate

aftercooling; air inlet temperature decreased to

200 F, although horsepower and BMEP both

increased; fuel consumption again decreased;

because turbocharging is also the same as in

column 2, maximum cycle pressure is also the

same, while cooler inlet air lowers thermal

loading.

Column 4 the same degree of aftercooling as

in column 3, but with a light turbocharging

boost; again bhp and BMEP increased, while

lowering fuel consumption; the higher boost

pressure brings considerably higher maximum

cycle pressures; but thermal load remains almost

unchanged from column 3.

Column 5 a very high degree of aftercooling

on the same amount of turbocharging boost as

column 4; Horsepower now stands at 214

114% more than the naturally aspirated engine;

BMEP is nearly 100 psi higher than that of thenaturally aspirated engine; fuel consumption is

more than 9% lower; maximum cycle pressure

has increased, but less than horsepower has;

thermal loading is only 13% greater than that of

the naturally aspirated engine, and is lower than

that of the lightly turbocharged, not aftercooled

engine in column 2.

Conclusion As BMEP is increased, fuel

consumption falls consistently. Mechanical

loadings due to cylinder pressures increase,

while thermal loadings rise slightly, then start to

decrease. This demonstrates that BMEP, with

little or no direct correlation to either

mechanical or thermal stresses, it is not an

indication of engine life.

Properly designed high BMEP engines may

have even better life expectancy than a

naturally aspiratedengine.

A high BMEP engine will have betterbhp-hr production (i.e. total amount of

work performed) than its low BMEP

counterpart.

A modern, naturally-aspirated, heavy-duty diesel

will live 10,000 hours between overhauls. For

example, a moderately blown version of the

same engine will produce 35 percent more

power for 8500 hours before overhaul, the blown

engine, at higher BMEP, has produced nearly 15

percent more bhp hours than the naturally-

aspirated engine, using only about 10 percent

more fuel to do it (less, per bhp-hr.).

Another way to look at it is that this moderately

blown engine would require only 7400 hours to

produce the same 1,000,000 hp-hrs and would

burn less fuel to do it. So, if an engine was

designed for that degree of turbocharging, it may

actually outlive a naturally aspirated engine.


BRAKE MEAN EFFECTIVE PRESSURE


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Engine torque can be measured using a

dynamometer, which is a device allowing an

operator to vary engine load. With the engine

running at full load speed, torque is measured

and plotted. The load is then increased slightly,

and the torque is measured, along with the drop

in speed; and that point is plotted. A further load

increase then produces further engine speed

reduction and torque increase, and another point

to graph. When enough points have been plotted,

we can connect them producing a lug torque

curve as shown below.

Because horsepower is a straight mathematical

derivation of the two quantities shown on the

graph (rpm and torque), we can calculate a

horsepower for each point on the torque curve,

and arrive at a corresponding lug horsepower

curve.

If fuel consumption is measured for each

loading, we can also produce a curve for this

data. Given in terms of the quantity of fuel

burned to produce one brake horsepower for one

hour, this data is called Brake Specific Fuel

Consumption (BSFC), as shown below.

The fuel consumed may be measured by weight

(pounds in the English system; grams in the

metric system) or by volume (gallons or liters).


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POWER CURVES

Torque

rpm

Torque

hp

rpm

Torque

0.5

0.4 BSFC

0.3

hp

rpm


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BMEP is also sometimes shown on such graphs

by calculating BMEP for each point on the

torque curve and plotting the resulting data.

For each engine setting, besides a lug torque

curve, there is also a starting torque or

acceleration curve. When a load is applied to

an engine which is operating considerably below

full load rpm, and the engine must then

accelerate carrying that load, a curve similar tothat shown below would be produced. This type

of loading is common in applications such as

road vehicles.

A completely different torque curve is produced

by an engine operating at or near full load rpm

before a load is applied. If the applied load is

equal to or less than that which the engine can

carry at that throttle setting, the governor opens

the rack enough to allow the engine to produce

the required power, and engine rpm remains

steady.

If, however, a load greater than full load is

applied, the engine will no longer be able to

maintain steady speed at that governor setting,and will begin to slow down, or lug.

Because volumetric efficiencies are somewhat

better at the slower speeds and frictional losses

are smaller, a greater torque can be produced at

lower speeds, so the increased load can be

carried.


POWER CURVES

Torque

0.5

BMEP

0.4 BSFC

0.3

hp

rpm

Torque

rpm

Lo-Idle

Acceleration

Torque

Full Load

Torque

rpm

Lo-Idle

Lug Torque

Full Load

Torque

rpm

Torque Rise


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A certain amount of torque, or rise, is normal

under lug conditions, but it can be substantially

increased by modifying the fuel and air systems.

With modern, medium-speed, turbocharged,

aftercooled diesels, this potential torque increase

is approximately 20 to 50 percent.

If the torque rise is steep enough, the engine

may develop more power at the lower speed,

because torque increases faster than speeddecreases.

The engine can support some overload, although

at a reduced speed. Should the applied load be

greater than that shown at peak torque, the

engine will rapidly slow down further, produce

less and less torque, and stall.

This type of loading, with the engine running

with a fixed throttle setting at or near full load,

is common in earthmoving equipment, such as

crawler tractors, track-type loaders, and many

shovel applications. It is also the normal loading

on generator sets.

By superimposing the acceleration and lug

curves, we can see the two different basic torque

curves common to all engines. Actually, an

infinite number of possible curves exist,depending on the engine speed at the start of

loading, and the throttle opening.


12

POWERCURVES

Torque

rpm

Modified Curve

Normal Curve

Torque

hp

rpm

Torque

rpm

Lo-Idle

Lug Torque

Full Load

Torque

rpm

Lo-Idle

Lug Torque

Acceleration

Torque

Full Load


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Another curve you will often see is called a

Pressure Time (PT) curve. The vertical axis

represents pressure; the horizontal axis

represents time.

Time in this case is measured in degrees of

engine crankshaft rotation rather than in seconds

or minutes. When the crankshaft has made one

full revolution, it has traveled 360 degrees.

When the piston is at its lowest point in the

cylinder, it is at Bottom Center (BC). As it starts

upward on the compression stroke, pressure

begins to rise, until it reaches a maximum whenthe piston is at Top Center (TC). Should that

cylinder fail to fire, pressure would drop off as

shown, and would again be zero at BC.

If the cylinder does fire:

1. At some point shortly before TC (usually

10-20 degrees of crankshaft rotation), fuel

injection begins. (This fuel is cold, and hasnot had time to mix with air, so it does not

immediately start to burn.) This period is

called ignition delay.

2. When the fuel does start to burn, heat is

generated, rapidly increasing the pressure

of the fuel-air mixture.

3. The peak pressure comes some few degrees

after TC. Although some of the fuel is still

burning at this point, the piston is moving

down so rapidly that volume increases faster

than the pressure can now increase, and

pressure starts to fall off.

The volume between the two curves is the network produced by combustion. A long-time

engineering goal is altering the shape of that

curve so that the volume under it is maximized

for a given cylinder pressure.


Pressure

Time

TCBC BC

Pressure

Time

Injection

Ignition Delay

TCBC BC

POWERCURVES

Pressure

Time

TCBC BC


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Understanding the basic concepts of how engines produce power is vital to successfully selling, buying,

operating, or maintaining an engine for any application. The more knowledge you have, the more youcontribute to ensuring an engines superior performance and reliability. Most of the terms associated

with engine power are easy to understand.


14

Some of the basic terms related to engine sizing

include:

Bore the diameter of each cylinder in an

engine

Stroke the distance a piston travels up and

down within a cylinder

Displacement the volume of air which the

piston displaces as it moves one stroke

Compression ratio the relationship between

the minimum and maximum volumes between

the piston crown and the cylinder head

(i.e. volume at BC divided by volume at TC)

Terms associated with engine power include:

Horsepower (hp) a measurement of engine

power

Torque the twisting force engines produce

Brake Mean Effective Pressure (BMEP)

the pressure in a cylinder required to produce

the same horsepower at the flywheel as actually

exists; is NOT indicative of engine life as many

believe

Lug a slowing of an engine occurring when

its load is greater than it can support at a

particular governor setting

Pressure-time curve a visual representation

of the pressure within the combustion chamber

during an engines cycle

ENGINE POWER CONCEPTS


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1997 Caterpillar Inc

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Conceptos de Potencia Motor